Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices. CFMs include implantable pacemakers, implantable cardioverter defibrillators (ICDs), and devices that include a combination of pacing and defibrillation including cardiac resynchronization therapy. The devices are typically used to treat patients using electrical therapy and to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include electrical leads in communication with sense amplifiers to monitor electrical heart activity within a patient, and often include sensors to monitor other internal patient parameters. Other examples of implantable medical devices include implantable insulin pumps or devices implanted to administer drugs to a patient.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs are able to detect abnormally slow heart rate, or bradycardia. The IMDs are also able to detect abnormally rapid heart rate, or tachyarrhythmia. Patients that use IMDs may be adversely affected by over-sensing of the sense amplifiers. Over-sensing occurs when sense-amplifiers incorrectly interpret sensed signals or noise as cardiac signal artifacts, such as P or R-waves indicative of atrial or ventricular heart contractions respectively. Over-sensing can result in a bradycardia device incorrectly inhibiting pacing therapy or a tachyarrhythmia device incorrectly delivering high energy shock therapy. Patients that use IMDs to treat tachyarrhythmia may also be adversely affected by under-sensing. Under-sensing occurs when electrical signals associated with tachyarrhythmia are not sensed by the device and therefore, high-energy shock therapy is not delivered. The present inventors have recognized a need for improved sensing of events related to cardiac activity.